The present invention relates to cooling tubes, electrode holders and electrodes for an arc plasma torch. The invention further relates to arrangements thereof and an arc plasma torch with such tubes, holders, electrodes, and arrangements.
A plasma is an electrically conductive gas consisting of positive and negative ions, electrons and excited and neutral atoms, and molecules, which is heated thermally to a high temperature. Various gases are used as plasma gases, such as mono-atomic argon and/or the diatomic gases hydrogen, nitrogen, oxygen or air. These gases are ionised and dissociated by the energy of an electric arc. The electric arc is constricted by a nozzle and is then referred to as a plasma jet.
The parameters of a plasma jet can be heavily influenced by the design of a nozzle and electrode. Such parameters of the plasma jet are, for example, the diameter of the jet, temperature, energy density, and the flow rate of the gas. In plasma cutting, for example, the plasma is constricted by a nozzle, which can be cooled by gas or water. In this way, energy densities of up to 2×106 W/cm2 can be achieved. Temperatures of up to 30,000° C. arise in the plasma jet, which, in combination with the high flow rate of the gas, make it possible to achieve very high cutting speeds on materials.
Because of the high thermal stress level on nozzles, nozzles are usually made from a metallic material, preferably copper, because of copper's high electrical conductivity and thermal conductivity. The same is true of electrodes, though electrodes are also commonly made of silver. A nozzle is often inserted into an arc plasma torch, called a plasma torch for short. The main elements of a plasma torch include a plasma torch head, a nozzle cap, a plasma gas conducting member, a nozzle, a nozzle holder, an electrode with an electrode insert, and, in modern plasma torches, a holder for a nozzle protection cap, and a nozzle protection cap. Inside the electrode, there is, for example, a pointed electrode insert made from tungsten, which is suitable when non-oxidising gases are used as the plasma gas, such as a mixture of argon and hydrogen. A flat-tip electrode, the electrode insert of which is made of hafnium, is also suitable when oxidising gases are used as the plasma gas, such as air or oxygen.
In order to improve the service life for a nozzle and an electrode, a cooling fluid is often used, such as water, though cooling may also be effected with a gas. For this reason, a distinction is made between liquid-cooled and gas-cooled plasma torches.
Electrodes are often made from a material with good electric and thermal conductivity, e.g. copper and silver or their alloys, and an electrode insert consisting of a temperature-resistant material, e.g. tungsten, zirconium or hafnium. For plasma gases containing oxygen, zirconium may be used. Because of its superior thermal properties, hafnium is, however, better suited, since its oxide is more temperature-resistant.
In order to improve the service life for an electrode, a refractory material is often introduced into the holder as an emission insert, which is then cooled. The most effective form of cooling is liquid cooling.
A plasma torch, can be configured with an electrode that is hollow in the interior and with a cooling tube inside. In Former East Germany Document DD 87 361, for example, water flows through the interior of the cooling tube, streams against the bottom of the electrode, and then flows back between the interior surface of the electrode and the exterior surface of the cooling tube.
The electrode often has a cylindrical or conical region extending inwards, with the cooling tube projecting beyond it. The coolant flows around this region and is intended to ensure a better exchange of heat between the electrode and the coolant.
Nevertheless, it is common for heating to occur at the electrode. This, when the apparatus is switched on for a long time, becomes apparent in the form of considerable discoloration of the electrode holder and rapid burn-back of the electrode insert.